Quantum computers promise to revolutionize computing and communications by harnessing the strange properties of quantum mechanics.
However, connecting distant devices is challenging because signals degrade rapidly over long distances and are prone to interference. Researchers from Princeton University have developed a new atomic device that enables high-fidelity transmission of information over fibre optic networks.
The key innovation is using a single erbium ion-implanted in a calcium tungstate crystal as a quantum repeater. The erbium ion emits light at an ideal wavelength for fibre optic transmission without needing conversion. This makes the system simpler and more robust compared to other approaches that emit visible light.
The device consists of the doped calcium tungstate crystal coupled to a tiny silicon channel etched in a J-shape. When stimulated with a laser pulse, the erbium ion emits a photon containing information encoded in its spin state. The silicon piece guides this photon out into an optical fibre with high efficiency. By collecting and interfering signals from many such nodes, quantum entanglement can be created between distant spins, enabling end-to-end transmission of quantum states.
After testing hundreds of materials, the researchers found calcium tungstate enabled the erbium ion to have long storage times and emit indistinguishable photons, which is critical for quantum interference. They demonstrated this by sending photons through a 22-mile fibre-optic loop. The strong suppression of unpaired photons at the output verified the erbium photon stream was of high fidelity.
While a major milestone, work remains to further improve storage times of quantum states in the erbium spins. The team is refining the calcium tungstate to reduce impurities that cause spin decoherence. Longer storage times will be needed before these quantum repeaters can be deployed in practical networks. However, this new device overcomes a major hurdle in transmitting signals over global distances.
The successful synthesis of materials science, photonics, and information theory exemplifies the multidisciplinary nature of technology research. This study provides a promising new architecture for achieving repeater networks, bringing us closer to the vision of a quantum internet where uncrackable communication links connect quantum computers across the planet.
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Reference: “Indistinguishable telecom band photons from a single erbium ion in the solid state” Salim Ourari, Łukasz Dusanowski, Sebastian P. Horvath, Mehmet T. Uysal, Christopher M. Phenicie, Paul Stevenson, Mouktik Raha, Songtao Chen, Robert J. Cava, Nathalie P. de Leon and Jeff D. Thompson, 30 August 2023, Nature.